The invention relates to a vehicle power supply system having two batteries (a main power source and an auxiliary power source).
A previously proposed vehicle power supply system having two batteries (2-battery system) is disclosed in, for example, JP-A-6-296332, JP-A-2001-186687 or U.S. Pat. No. 6,275,001.
The system set forth in JP-A-6-296332 includes a main power storage means and a backup power storage means, which are connected to each other through a DC/DC converter. Switching of the DC/DC converter is controlled in such a manner that regenerative energy (electric power), which is obtained during deceleration of the vehicle, is stored in the backup power storage means, and the stored electric power is supplied from the backup power storage means to the electrical loads of the vehicle at time (e.g., time of accelerating the vehicle, time of steady driving of the vehicle or time of idling the engine) other than the time of decelerating the vehicle at a higher priority over the main power storage means.
The system set forth in JP-A-2001-186687 has a power supply network battery and a starter battery. The power supply network battery is connected to a generator, and the starter battery is connected to a starter. These batteries are connected to each other through a DC/DC converter and a switch. Here, the switch is provided in parallel with the DC/DC converter.
The system set forth in U.S. Pat. No. 6,275,001 has a first battery, a second battery and a generator. The first battery supplies electric power to a starter at the time of starting the engine, and the second battery supplies electric power to electrical loads mounted on the vehicle. The generator charges the first battery and the second battery. With this construction, it is possible to supply power to the electrical loads from the second battery while power is supplied from the first battery to the starter to start the engine. Accordingly, there is no momentary stop of electrical loads (e.g., a navigation system), which would be otherwise caused by a voltage drop of the first battery due to flow of a large current (surge current) in the initial stage of supplying electric power to the starter, and the voltage to the electrical loads can be guaranteed by the second battery.
However, in the system set forth in JP-A-6-296332, there are the following disadvantages. Firstly, regenerative energy generated by an alternator during deceleration of the vehicle is stored in the backup power storage means through the DC/DC converter. Thus, due to the presence of the DC/DC converter between the alternator and the backup power storage means, the recovery efficiency is decreased to cause deterioration of fuel consumption. Also, since power stored in the backup power storage means also goes through the DC/DC converter at the time of supplying electric power to the electrical loads of the vehicle, the power supply efficiency is decreased. Furthermore, the operating voltage of the DC/DC converter is applied from the main power storage means. Thus, if the main power storage means becomes inoperable (e.g., at the time of battery death), energy recovery during deceleration becomes impossible, and also electric power cannot be supplied from the backup power storage means to the electrical loads. Thus, the reliability of the system is largely decreased.
In the system of JP-A-2001-186687, there are the following disadvantages. Firstly, since power generated by the generator is supplied to the starter battery through the DC/DC converter, energy efficiency is poor. Also, at the time of performing high power generation (e.g., regenerative power generation during vehicle deceleration) through use of the generator, large voltage fluctuations act on the power supply network battery and the electrical loads. Thus, troubles, such as flickering of lights and wiper speed fluctuations, occur. A safety measure for counteracting with a failure of one of the power supply network battery and the starter battery is not implemented for important devices (e.g., safety devices, such as an electric braking device, electric power steering device), which are among the electrical loads mounted on the vehicle and need stable electric power supply. For example, when the power supply network battery has shorted to the ground side, it is not possible to isolate the power supply network battery and to supply power to the important devices from the starter battery.
In the system set forth in U.S. Pat. No. 6,275,001, there are the following disadvantages. That is, at the time of starting the engine, the respective roles of the first battery and the second battery are limited, and engine starting is carried out with the first battery only. Thus, engine starting characteristics are not improved in comparison to an ordinary 1-battery vehicle power supply system.
For example, in a case of a vehicle equipped with an idle stop system (an eco-run system that automatically stops an engine at the time of stopping the vehicle and then automatically controls restarting of the engine based on, for example, a signal indicative of releasing of a brake), it is desirable to restart the engine within a short period of time at the time of moving the vehicle, for example, at the blue traffic signal after the automatic stopping of the engine at, for example, the intersection. However, when power supply to the starter is carried out using the first battery only, it is difficult to shorten the starting time, and thus a significant advantage cannot be expected from the system set forth in U.S. Pat. No. 6,275,001 when the system is implemented in the eco-run system.
The present invention addresses the above disadvantages, and it is a first objective of the invention to provide a more reliable vehicle power supply system, with which it is possible to efficiently recover energy regenerated during deceleration of a vehicle and also to supply a stable voltage to electrical loads.
It is a second objective to provide a vehicle power supply system, with which it is possible to carry out recovery of energy regenerated during deceleration of a vehicle and supply of power to electrical loads efficiently.
It is a third objective to provide a vehicle power supply system, with which it is possible to guarantee a voltage and furthermore shorten starting time of an engine.